2.1 Biology and Life Cycle of Coccidial Protozoa
Coccidial protozoa may infect and inhabit the gastrointestinal tract of most species of warm-blooded animals, most often and most seriously when the animals are penned or in concentrated groups. For the most part infection occurs with coccidial species specific to the animal host species and more than one coccidial species may infect a given host animal species. For example, seven major coccidial species are known to infect the chicken.
The life cycle of the coccidial parasite involves several complex stages. Under natural conditions the cycle begins with oral ingestion by the host animal of excrement from another animal of the same species having sporulated oocysts dispersed therein. The sporulated oocyst shell then breaks down by action of the digestive system of the host animal. For example when the host animal is a chicken the outer shell of the sporulated oocyst is broken down mechanically to liberate first the sporocysts and finally biochemically to liberate sporozoites which are the ultimate infective organism. Other stages in the cycle which follow redevelopment within about 7 days include schizont I, merozoite I, schizont II, merozoite II, gametocyte, gamete and completion of the cycle with oocyst formation.
More descriptively, in the instance of the chicken, natural infection with one of the important species of Eimeria, i.e., Eimeria tenella begins with ingestion of an overwhelming amount of the sporulated oocysts. Each sporulated E. tenella oocyst contains four sporocysts and each sporocyst contains two sporozoites. In the stomach, the sporulated oocysts rupture, freeing the sporocysts. The sporozoites are liberated from the sporocysts by enzymatic action and pass down the digestive tract. Each of the sporozoites eventually reach the cecal pouches where they enter epithelial cells of the cecal mucosa and start an asexual multiplication cycle. The details of the completion of the life cycle referred to above are well known in the art.
The rationale of vaccination with live sporozoites, in the present invention, is to set up a low level of infection (subclinical) during the first few days of life of the animal, e.g., a chicken, without slowing down the performance of the animal in terms of growth. The subclinical infection, thus established induces immunity, while the host, e.g., chicken, has the advantages of passive protection afforded by maternal antibodies. The maternal antibodies also help to down regulate infection, thus helping to decrease unnecessary parasite buildup in animal houses, which is critical for the success of a vaccine since the degree of severity of disease is directly proportional to the numbers of sporulated oocysts ingested by the host from its environment. Under field conditions chickens, e.g., have their maternal anitbodies reduced to very low levels by the third week and hence become susceptible to the parasite, if their immune system is not primed by vaccination. Without vaccination the challenge encountered by hosts is frequently devastating, especially to concentrated populations of animals resulting in death or illness and/or poor development or growth quality and subsequent economic loss to the animal grower.